On dec/xx/97 at XA50 MST, I was the acting captain on a cessna citation sii, departing from rock springs, wy, to slc. The WX in slc was above minimums in snow for ILS approachs the entire morning and slowly improving. I was initially cleared to cartr intersection to hold as published with approximately a 30 min efc time -- which was extended due to snow removal problems at slc. Our fuel quantity was never a problem and we could have held for more than an hour longer than we did with reserves. I believe we held for close to an hour in all. During holding while in a couple of the course reversals the pilot's HSI #1 heading flag appeared momentarily -- perhaps for a second, and then returned to normal. This is a contributing factor. From holding at cartr to being vectored to downwind for runway 34L the heading flag never appeared. Upon turning left for a vectored heading of 360 degrees from 160 degrees to intercept the localizer the heading flag again appeared but the HSI #1 compass seemed to be slewing normally with RMI #1 and HSI #2. With the heading flag the autoplt command bars were lost and the yaw dampener dropped off-line. I had been flying by hand during the entire vector sequence. As this malfunction was occurring my task saturation was high -- flying the aircraft and ascertaining what for sure had failed. In the seconds from the point when the flag appeared to when I rolled out near the assigned heading I noticed a split of 40- 50 degrees from RMI #1 and HSI #2. The approach controller caught this simultaneously and made his right turn instruction. In actuality I had turned the plane to a heading of 310-320 degrees. This is where the TCASII was alerted on an aircraft that was following us in trail. There were several seconds of confusion of exactly what heading the aircraft I was flying was on until a xchk with the magnetic compass confirmed the HSI #2 to be accurate. I immediately began a turn back to the right using RMI #1 as my heading reference, intercepted the localizer and completed the approach using raw data. Being that the pilot's HSI and RMI #1 seemed to slew identically during the first seconds of the turn after the flag appeared I was deceived assuming that a glitch had again occurred as earlier in holding. Once that flag had shown if I had assumed a hard failure and used the RMI #1 as a reference no incident would have happened. I've already discussed this with the slc FSDO and no enforcement will be taken but I felt ASRS might appreciate hearing of this incident. They seemed sympathetic that this failure happened at the worst possible time during the approach. Lesson learned.

Narrative: ON DEC/XX/97 AT XA50 MST, I WAS THE ACTING CAPT ON A CESSNA CITATION SII, DEPARTING FROM ROCK SPRINGS, WY, TO SLC. THE WX IN SLC WAS ABOVE MINIMUMS IN SNOW FOR ILS APCHS THE ENTIRE MORNING AND SLOWLY IMPROVING. I WAS INITIALLY CLRED TO CARTR INTXN TO HOLD AS PUBLISHED WITH APPROX A 30 MIN EFC TIME -- WHICH WAS EXTENDED DUE TO SNOW REMOVAL PROBS AT SLC. OUR FUEL QUANTITY WAS NEVER A PROB AND WE COULD HAVE HELD FOR MORE THAN AN HR LONGER THAN WE DID WITH RESERVES. I BELIEVE WE HELD FOR CLOSE TO AN HR IN ALL. DURING HOLDING WHILE IN A COUPLE OF THE COURSE REVERSALS THE PLT'S HSI #1 HDG FLAG APPEARED MOMENTARILY -- PERHAPS FOR A SECOND, AND THEN RETURNED TO NORMAL. THIS IS A CONTRIBUTING FACTOR. FROM HOLDING AT CARTR TO BEING VECTORED TO DOWNWIND FOR RWY 34L THE HDG FLAG NEVER APPEARED. UPON TURNING L FOR A VECTORED HDG OF 360 DEGS FROM 160 DEGS TO INTERCEPT THE LOC THE HDG FLAG AGAIN APPEARED BUT THE HSI #1 COMPASS SEEMED TO BE SLEWING NORMALLY WITH RMI #1 AND HSI #2. WITH THE HDG FLAG THE AUTOPLT COMMAND BARS WERE LOST AND THE YAW DAMPENER DROPPED OFF-LINE. I HAD BEEN FLYING BY HAND DURING THE ENTIRE VECTOR SEQUENCE. AS THIS MALFUNCTION WAS OCCURRING MY TASK SATURATION WAS HIGH -- FLYING THE ACFT AND ASCERTAINING WHAT FOR SURE HAD FAILED. IN THE SECONDS FROM THE POINT WHEN THE FLAG APPEARED TO WHEN I ROLLED OUT NEAR THE ASSIGNED HDG I NOTICED A SPLIT OF 40- 50 DEGS FROM RMI #1 AND HSI #2. THE APCH CTLR CAUGHT THIS SIMULTANEOUSLY AND MADE HIS R TURN INSTRUCTION. IN ACTUALITY I HAD TURNED THE PLANE TO A HDG OF 310-320 DEGS. THIS IS WHERE THE TCASII WAS ALERTED ON AN ACFT THAT WAS FOLLOWING US IN TRAIL. THERE WERE SEVERAL SECONDS OF CONFUSION OF EXACTLY WHAT HDG THE ACFT I WAS FLYING WAS ON UNTIL A XCHK WITH THE MAGNETIC COMPASS CONFIRMED THE HSI #2 TO BE ACCURATE. I IMMEDIATELY BEGAN A TURN BACK TO THE R USING RMI #1 AS MY HDG REF, INTERCEPTED THE LOC AND COMPLETED THE APCH USING RAW DATA. BEING THAT THE PLT'S HSI AND RMI #1 SEEMED TO SLEW IDENTICALLY DURING THE FIRST SECONDS OF THE TURN AFTER THE FLAG APPEARED I WAS DECEIVED ASSUMING THAT A GLITCH HAD AGAIN OCCURRED AS EARLIER IN HOLDING. ONCE THAT FLAG HAD SHOWN IF I HAD ASSUMED A HARD FAILURE AND USED THE RMI #1 AS A REF NO INCIDENT WOULD HAVE HAPPENED. I'VE ALREADY DISCUSSED THIS WITH THE SLC FSDO AND NO ENFORCEMENT WILL BE TAKEN BUT I FELT ASRS MIGHT APPRECIATE HEARING OF THIS INCIDENT. THEY SEEMED SYMPATHETIC THAT THIS FAILURE HAPPENED AT THE WORST POSSIBLE TIME DURING THE APCH. LESSON LEARNED.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.